1. Field of the Invention
The present invention relates generally to digital computers and, more particularly, to a digital computer adapted for low power operation while playing an audio CD.
2. Description of the Prior Art
Portable computers (i.e., notebook, laptop, palmtop and the like) from major original equipment manufacturers such as Toshiba, Compaq, Dell, IBM and others offer CD-ROM drives as either standard or optional devices. Notebook, laptop, palmtop computers are aimed at the mobile computer user who needs or wants to take work home from the office or on a business trip. An added benefit of CD-ROM equipped portable computers is the opportunity to enjoy periods of relaxation and pleasure by playing audio tracks from standard music CDs. In the ensuing discussion, the term notebook computer will be understood to apply also to laptop, palmtop and other portable, battery powered computers.
The Windows operating system's media player or third party audio application can play back standard audio CDs on a portable computer. However the simple function of playing an integral audio CD-ROM requires that the entire notebook system be powered for the duration of the audio play back. This causes excessive drain on the notebook's battery power system, unnecessarily consuming battery energy better saved for CPU intensive use such as word processing and spreadsheet analysis.
Conventional laptop and notebook computers typically have several power down modes. They can be powered down such that the CPU is almost completely off, with the state of the CPU saved on a hard drive. A very low power portion of the CPU or an auxiliary circuit (e.g. keyboard controller) is typically used to recognize when a key is pressed. The system then reactivates normal power to allow the CPU to retrieve the stored machine state from the hard drive thereby restoring the computer into an operating mode. Some well known power saving modes are called sleep mode, suspend mode and the like.
Consequently, a modern energy efficient computer will, over time, operate in several different power management regimes. For example, if a portable computer is being used in an office environment where electrical power consumption is an insignificant concern, then the computer user may want the computer to provide the highest performance and availability possible. Conversely, if the computer is being operated on battery power where there is no convenient source of electrical energy, then the computer user may want to choose a power management regime for the computer that will maximize the time the computer operates without recharging its batteries, even though performance and availability may be noticeably reduced.
To facilitate controlling electrical power consumption in personal computers, Intel Corporation, Microsoft Corporation, and Toshiba Corporation have jointly established an Advanced Configuration and Power Interface Specification (“ACPI Specification”). The ACPI Specification Revision 1.0 of Dec. 22, 1996, Copyright 1996 Intel Corporation, Microsoft Corporation, Toshiba Corporation, establishes both a set of five (5) Global System States G3—Mechanical Off, G2/S5—Soft Off, G1—Sleeping, G0—Working, S4—Non-Volatile Sleep, and a set of four (4) Device Power States D0—Fully On, through D3—Off. The ACPI Specification defines the Global System States as follows.                G3 Electrical power is mechanically turned off.        G2/S5 Electrical power is turned on but the computer consumes a minimal amount of power by not executing either user or system computer programs, and the system's context is not preserved by hardware.        G1 Electrical power is turned on, the system's context is preserved by hardware or system software, but user computer programs are not being executed.        G0 Electrical power is turned on and user computer programs are executed. In the G0 state, devices such as hard disk drives, CD-ROM drives, floppy diskette drives, etc are dynamically turned on and off as needed.        S4 Electrical power may either be turned off, i.e. Global State G3, or turned on with the computer consuming a minimal amount of power, i.e. Global State G2/S5, while system context is preserved in a non-volatile storage file before entering either the G3 or G2/S5 state, thereby permitting the computer to be restored to its prior operating state, i.e. G1 or G0.        
The ACPI Specification further defines Device Power States as follows.                D0 The device is completely active and responsive, and consumes the most electrical power.        D1 A lower power state that is defined for different types of devices which preserves more device context than the yet lower power state D2.        D2 An even lower power state than D1 that is again defined for different types of devices, and which preserves less device context than state D1.        D3 Electrical power is fully removed from the device, device context is lost, and system software must reinitialize the device when it is turned on again.        
The different computer operating modes and associated power management regimes described above are each characterized by a unique power demand (i.e., current drain) from the battery power supply. This is an important feature both in design of portable computer systems, and in marketing them as well. Great attention is focused on minimizing the power demand for each of the different Global and Device operating modes. Thus, the power demand characterizing each power management regime is a critical factor to be considered for portable computers, particularly one that includes a CD-ROM drive for playing audio CDs.
In implementing conventional computer power management strategies, a power management routine (“PMR”) executed by the CPU must periodically monitor peripheral devices to assess whether a peripheral device's operation may be suspended. Similarly, if it becomes necessary to access a peripheral device whose operation has been suspended such as in Device Power modes D1–D3, the PMR must resume that peripheral device's operation. Generally, suspending the operation of a peripheral device and resuming its operation respectively require that the PMR executed by the CPU perform a unique sequence of operations in turning off electrical power to a peripheral device, and in turning electrical power back on. Writing a computer program that detects a need to execute a power-on or a power-off sequence of operations for a peripheral device is a cumbersome task.
Previous portable computers that include a CD-ROM use PMR functions to minimize battery drain. However, if CPU operation has been suspended to save electrical power, the computer can essentially do nothing. Therefore, in the minimal power drain mode, the CPU cannot use the windows operating system's media player or third party audio application to play audio CDs.
A significant power drain in portable computers occurs in the conventional LCD monitor. Typically, 60 to 70% of the power consumed by a notebook is consumed by the display. Thus even if a computer's devices, including even perhaps the CPU, were in a lower power state, i.e., one of the lower Device Power States D1–D3 for power savings during CD-ROM play only, the need to use the normal LCD to display CD-ROM status and the music playing status would itself impede significantly reducing power consumption.
For the reasons described above, it is apparent that a disadvantage of present portable computers for playing audio CDs is that some portion of the computer system must remain energized state to detect key actuation and then to restore power or activate a power restore function of the CPU and associated peripherals (e.g. hard drive, keyboard controller, display, etc.). At times when a portable computer is being used during travel, or when line power is otherwise unavailable, the user may wish to play some audio CDs. Given the limited battery life of most portables, e.g., 3 to 5 hours of use, the user may have to choose to forego using the CD-ROM capability for very long, out of fear that the notebook will not be functional for needed work or communication.